This invention relates to facsimile systems comprising a transmitter, a receiver and a communications network therebetween. More particularly, this invention relates to a system wherein a document is scanned in a facsimile transmitter to generate electrical information-bearing signals representing the dark-light variations in the document being scanned. These information-bearing signals are then transmitted over the communications network to a facsimile receiver where the information-bearing signals are converted to marks or images on a copy medium so as to form a copy which is a reasonable facsimile of the original document.
Typically, the communications network comprises ordinary telephone lines which have a limited bandwidth capacity. In order to stay within this bandwidth capacity, the scan rate must be maintained so that the maximum frequency of light-dark variations is maintained within the bandwidth.
Heretofore, various line-skipping facsimile schemes have been proposed to permit a rapid facsimile transmission by utilizing a fast scanning rate over those portions of a document or image which are devoid of information. Systems of this type are disclosed in U.S. Pat. Nos. 3,286,026--Greutman et al, 3,128,338--Teacher et al and 2,287,413--Bruce et al. In the systems disclosed in these patents, separate and distinct signals are generated and transmitted to adjust the scanning rate of the document or image. Accordingly, a portion of the available bandwidth of the communications network must be utilized for scanning rate control. This in turn reduces the available bandwidth for transmission of the information-bearing signals unless a separate communications link is provided for scanning control. Moreover, such systems require initial additional circuitry for the transmission and reception of the separate and distinct scanning rate control signals.
U.S. Pat. Nos. 3,448,207--Green et al and 3,201,512--Mason et al disclose facsimile systems which are capable of operating at a fast scanning rate in response to the sensing of information voids by lead fiber optics positioned in advance of the read or video fiber optics which detect video information for transmission. In both the Green et al and Mason et al patents, fiber optics are spaced a good distance from and communicate with the document through lenses. If the fiber optics were to be placed in direct communication with the document as disclosed in copending application Ser. No. 412,989, filed Nov. 5, 1973 now U.S. Pat. No. 3,956,587, considerable difficulty might be encountered in distinguishing between shadows created by the edge of the document being scanned and/or the clamping mechanism utilized to hold the document in place. As set forth in the aforesaid copending application, the use of fiber optics in direct communication with a document is extremely advantageous from an economy standpoint in addition to producing a very high degree of facsimile resolution.